1. Field of the Invention
The present invention relates to an image pickup system of a rolling shutter type and an image pickup method for sequentially performing exposure of an image pickup device for each of horizontal lines.
2. Description of the Related Art
In an image pickup apparatus, a CCD image sensor has been widely used. However, in recent years, a CMOS image sensor (hereinafter simply referred to as CMOS) has also started to be widely used. The CMOS consumes low power. Peripheral circuits can be formed on the same substrate in the CMOS. In a field of an electronic endoscope, an endoscope system using the CMOS has started to be proposed.
Incidentally, in the CMOS, in general, a rolling shutter type in which readout of pixel signals is performed in order for each one horizontal line is adopted.
Examples of an image pickup apparatus using such a CMOS of the rolling shutter type include an image pickup apparatus described in Japanese Patent Application Laid-Open Publication No. 2009-124260. Application examples of the CMOS in an endoscope apparatus include an endoscope apparatus described in Japanese Patent Application Laid-Open Publication No. 2011-30985.
Incidentally, in the case of the rolling shutter type, a state in which a line during exposure and a line during non-exposure are present at the same point of time occurs. When flush light emission (glint light emission) is performed at such a point of time, a line on which an object illuminated by illumination light is exposed and a line on which the object is exposed without being illuminated by the illumination light are generated.
Therefore, in the techniques described in Japanese Patent Application Laid-Open Publication No. 2009-124260 and Japanese Patent Application Laid-Open Publication No. 2011-30985, the apparatus is controlled to irradiate illumination light in an exposure period common to all lines (an all line exposure period or a global exposure period) to perform photographing.
It is conceivable to set a light amount change relatively gentle with respect to timing of image pickup (about 10 Hz at the highest). Such a gentle light amount change is performed in, for example, a light source apparatus including a light adjustment mechanism such as a mechanical diaphragm mechanism.
It is also conceivable to gently subject a light source (e.g., an LED light source) to current light adjustment (e.g., at about 10 Hz) instead of the gentle light amount change by the mechanical diaphragm mechanism.
For example, Japanese Patent Application Laid-Open Publication No. 2007-318581 describes an image pickup apparatus including an image pickup device such as a CMOS, image pickup device drive controlling means for controlling exposure of the image pickup device and readout driving in the rolling shutter type, frame cycle controlling means for controlling a frame cycle, a photographing auxiliary light source configured by an LED light or the like, and auxiliary light source drive controlling means for controlling the photographing auxiliary light source in PWM driving. In a technique described in the publication, in synchronization with an exposure start period for each of respective lines by a rolling shutter, the LED light is caused to perform pulse light emission and a light emission pulse is subjected to PWM control, whereby a light emission amount is adjusted (line light adjustment: see, for example, FIG. 3 and the like of the publication).
The technique described in Japanese Patent Application Laid-Open Publication No. 2007-318581 is a technique for subjecting the light emission pulse to PWM light adjustment for each one horizontal line period. Therefore, further, it is possible to use current light adjustment together with the PWM light adjustment.
Incidentally, in general, one horizontal line period is about several ten its. On the other hand, in a large-power LED driver used in an endoscope light source, generatable minimum pulse width is about several μs.